Cannabis-like chemicals in the brain play a key role in erasing nasty memories, a study has found.

Researchers in Germany have found that cannabinoids, produced naturally in the brain, help to manage fear.

They believe that a lack of these chemicals may explain why some people have difficulty forgetting painful events and suffer from post-traumatic stress disorder or phobias.

The finding might have implications for treating anxiety disorders in humans

Dr Pankaj Sah, Australian National University

The researchers suggest that new drugs to increase the production of cannibanoids in the brain could help these patients to forget their painful memories.

Doctors at the Max Planck Institute of Psychiatry in Munich made their discovery in laboratory tests.

Mutated mice

They bred two groups of mice. The first was engineered to produce more cannabinoids while the second was engineered to produce fewer of these chemicals.

They then subjected the mice to a series of tests. These involved a loud tone followed by a light electric shock to the foot.

They found that mice which produced fewer cannabinoids froze when they heard the tone, anticipating an electric shock.

The mice which produced more cannabinoids appeared to forget to associate the tone with the electric shock and didn't freeze.

The authors suggested their findings showed that cannabinoids played a key role in erasing painful memories.

They added that their results could one day be used to develop new treatments for people who have difficulty forgetting painful events.

These include patients with post-traumatic stress disorder, phobias or anxiety disorders.

Writing in the journal Nature, they said: "Overall our findings suggest that the endogenous cannabinoid system could represent a therapeutic target for the treatment of diseases associated with inappropriate retention of aversive memories or inadequate responses to aversive situations such as post-traumatic stress disorders, phobias, and certain forms of chronic pain."

However, they warned that cannabis itself would not have the same effect because it overflows the brain and is not
specific enough to extinguish the unpleasant memory.

Dr Pankaj Sah, a neuroscientist at the Australian National University in Canberra, welcomed the study.

In an accompanying article in Nature, he said: "The finding might have implications for treating anxiety disorders in humans."

He added that the study may explain why some people with mental illness turn to cannabis.

"It may be a form of self medication for the sometimes extreme anxiety that these people experience," he said.